Nanometre thick size films (about 2 nm thick) that were plasma deposited using a mixture of polymerizing and non-polymerizing gases on a PEEK polymer surface resulted in a remarkable autohesive strength (up to 10 fold when compared with the untreated control). Experimental results on autohesion of semicrystalline PEEK surfaces treated with plasma immersion ion implantation and deposition (PIIID) showed exceptionally strong autohesive bonds in lap-shear testing. Electron spin resonance (ESR) showed that the radical concentration increased linearly with plasma bias voltage. X-ray photoelectron spectroscopy (XPS) measurements showed no correlation between autohesive bonding strength and the concentration of oxygen and nitrogen elements and or the concentration of C-O functional groups on the surface. Contact angle measurements showed that surface energy also showed no correlation. Bond strength values are linked with the percentage of the sp3 components of the C Is region fitting in XPS spectra. There was strong relationship between the plasma treatment bias voltage and the plasma generated free radical density in the deposited thin film. Bond strength values are also correlated with the percentage of the sp3 components of the C Is region fitting in XPS spectra. Free radicals induced covalent bonding might be suggested as the major contributor that leads to that the remarkable increase of the adhesion strength. (C) 2013 Elsevier B.V. All rights reserved.